Elevator door systems generally consist of single or dual sliding doors which are normally powered to provide for automatic opening and closing. The actual door panels slide open in a horizontal plane to provide access and closed to provide security to the elevator passengers. Traditionally, these automatic operation systems have been powered by an electric motor which provides rotational torque, as shown in FIG. 1. A mechanical linkage system is there shown that converts the rotational force produced by the motor into a linear force required to move the door. Two common forms of mechanical linkages used include a two-bar linkage and a threaded lead screw.
The concept being advanced by the present invention is to replace the combination of rotational motor and linkage with a linear electric motor. Linear electric motors have been suggested in the prior art for operating sliding doors and elevator doors.
For example, U.S. Pat. No. 3,462,883 shows a sliding door actuated by a linear induction motor having a control circuit including a plurality of switches sequentially actuated by the door moving in either direction for effecting variable deceleration. Speed sensing is also used to modify the deceleration control.
U.S. Pat. Nos. 4067,144 and 4,090,113 disclose a method of driving a door of an automatic door assembly by a linear motor mounted within the automatic door assembly. The door is driven by a normal propulsion force which is augmented during a final portion of the stroke of the door, thereby overcoming the reaction force of cushioning devices provided near the ends of the door stroke.
U.S. Pat. No. 3,872,622 discloses a linear motor for driving a pair of sliding doors, wherein the stator is fixed relative to the frame of the doors and its armature connected to a pulley and rope assembly, which is movable therewith to effect opening of the doors. Means are provided for adjusting the axis of rotation of the pulleys in three planes to adjust the tension on the rope and adjust the alignment of the pulleys.
Linear motors are known for various types of doors, as shown, for example, in U.S. Pat. Nos. 5,134,324; 4,858,452; 4,188,552; 3,793,944; 4,365,442; and 3,708,915.
Various speed control and control circuits are known for linear motors, such as shown in U.S. Pat. No. 3,891,907 and British Patent Specification 1,148,144.
However, inasmuch as it is desirable to place the linear motor at a top or bottom end of the door to be actuated, there will be developed an undesirable torque about the doors due to the moment arm for the horizontal force. This moment arm, which is the distance between the point of horizontal force application and the door unit center of gravity, remains constant over the door travel distance. This undesirable torque may be substantial and for a rapidly accelerating door will produce rotation or oscillation of the door about its center of gravity. As can be seen in FIG. 1, the prior art solves this problem for high-acceleration doors by placing the holding point somewhere near the middle of the door or close to the center of gravity so as to minimize a rotational torque problem that would otherwise arise.
In addition to the desirable horizontal force, the linear motor also produces an attractive force between the primary and secondary of the motor. In previous art unrelated to door systems, for example, a linear motor for lifting an elevator in the hoistway, this force was opposed by rollers or bearings located on the primary parts, thus maintaining the air gap required between the secondary and primary components of the motor.